When the ball on the left is pulled to the side and released, it will swing back and collide with the ball on the right. Given that both balls have the same mass and are rigidly touching, the collision will transfer the momentum from the first ball to the second.
When the left ball (let's call it Ball A) swings back and hits the right ball (Ball B), Ball A will come to a stop, and Ball B will swing to the right, moving away from Ball A. This is a classic example of an elastic collision for objects of equal mass.
Therefore, the correct response is:
The ball on the left will stop, and the ball on the right will swing to the right.